How to copy the vertex position to the inbetween values of the shapekey? (Not Basis)
If you click the drop-down arrow in the Shape Keys panel there is an option for 'New Shape From Mix' - this creates a new shape key from the mix of all your existing shape keys and values.
Simply set your shape key 'values' to set 0.5 for the key you want to get the 'half-way' position of and select 'New Shape From Mix'. This will create a new shape key that produces that distortion when fully set.

You can use the same method to create a new shapekey from a mix of any amount of any number of your existing shape keys.